1. Field of the Invention
This invention relates to a method of making glass in which a molten glass stream flows through a chamber between opposed sidewalls, wherein the marginal edge portions of such a molten glass stream are selectively heated to control the temperature profile thereacross.
2. Discussion of the Technical Problem
In the production of flat glass, glass batch materials are melted in a glass melting furnace and the molten glass is caused to flow downstream through a conditioner region and a canal region to a glass forming chamber. Within the glass forming chamber the molten glass is commonly supported upon a pool of molten metal, cooled, and acted upon to produce a stable ribbon of glass of the desired thickness, width, and optical quality.
Due to heat losses through the sidewalls of the conditioner region, canal region, and glass forming chamber, the stream of molten glass commonly exhibits a transverse temperature profile, i.e., a temperature profile across the width of the stream, in which the central portions of the stream are maintained at a higher temperature than marginal edge portions thereof. In the conditioner region and canal region of the system such a transverse temperature profile may produce undesirable flow characteristics and nonuniform erosion of the refractory members e.g., a threshold and/or tweel, in contact with the molten glass stream. In the glass forming chamber where attenuating forces are commonly exerted upon the stream, such a transverse temperature profile may produce undesirable thickness variations and optical distortions in the final product. These undesirable effects become more pronounced as the width of the respective chambers is increased, due to the more pronounced temperature gradient which tends to result.
U.S. Pat. No. 3,445,214 to Ormesher teaches facilities by which localized areas of a ribbon of glass supported upon a pool of molten metal may be heated. Generally, electrical currents are passed from an overlying electrode through the thickness of the glass ribbon to the underlying molten metal pool, which serves as the opposite electrode, to minimize heat losses from the marginal edges of the glass ribbon. The patent is limited in its applicability, however, to relatively high temperature regions of a glass forming chamber, where the glass is sufficiently electrically conductive while it is supported on a molten metal pool.
U.S. Pat. No. 3,533,773 to Fujimoto et al. teaches facilities for heating the marginal edge portions of a stream of molten glass which include a graphite electrical heating element which may be suspended over a marginal edge portion of the stream within a canal region or a glass forming chamber. However, because the heating element directs heat both toward and away from the glass to be heated in equal proportions, its operating efficiency is diminished. Additionally, the heating element is limited in its capacity to counter a transverse temperature profile having a smooth gradient of temperature change from center to marginal edge of the stream of molten glass, because it generally provides only a single, uniformly powered heating zone along the marginal edge portion of the stream, rather than a plurality of independent heating zones spaced between a sidewall and the center portion of the stream.
U.S. Pat. No. 3,973,940 to Edge et al. teaches a method of delivery of a stream of molten glass onto a molten metal pool which provides for substantially uniform flow rates across the width of the stream of molten glass. The transverse temperature profile of the stream of molten glass is controlled in the canal region by a plurality of transversely aligned heating elements. The heating elements disclosed are vertically suspended, generally U-shaped elements which are controlled in response to the tweel position to maintain the relative marginal and central stream flow rates within a prescribed range. The heating elements are limited, however, because the heat which is generated is not effectively directed toward selected portions of the stream of molten glass which are to be heated. It would be advantageous to have a method of and an apparatus for selectively and efficiently heating a marginal edge portion of a stream of molten glass, to diminish nonuniform transverse temperature profiles existing in the stream.